With growing concerns about global environmental issues and energy issues, solar energy is attracting attention as a clean, sustainable energy, with photovoltaic power generation becoming a focus of attention as a new scheme for electricity supply.
When using solar cells in outdoor environment such as on the roof of a building, they are generally interconnected and encapsulated to form a module. Solar cell modules are broadly classified into two groups: crystalline solar cell modules and thin-film solar cell modules. The crystalline solar module is generally manufactured through, for example, a lamination process in which a protection film for solar cell module (front protective member), an encapsulant sheet for solar cells, crystalline solar cells formed of polycrystalline silicon or mono-crystalline silicon, an encapsulant sheet for solar cells, and another protection film for solar cell module (back protective member) are sequentially stacked and the module is laminated under heating in vacuum condition.
The thin-film solar cell module is generally manufactured through, for example, a lamination process in which thin-film solar cell prepared by forming a ultra-thin (several micrometers) amorphous silicon or crystalline silicon film onto glass or the like, an encapsulant sheet for solar cells, and a protection film for solar cell module (back protective member) are sequentially stacked and the module is laminated under heating in vacuum condition.
Solar cell modules manufactured in this way are weather-resistant and thus are suitable for use in outdoor environment such as on the roof of a building.
Heretofore, ethylene-vinyl acetate (EVA) copolymer has been widely used as the material constituting an encapsulant sheet for solar cells (encapsulant material for solar cells) for the requirements for high transparency and high flexibility (see, e.g., Patent Literature 1). When EVA is used as encapsulant material for solar cells, the material is typically subjected to crosslinking to obtain sufficient heat resistance. Crosslinking treatment, however, takes a relatively long time (0.2 to 2 hr or so) and thus reduces the production rate or productivity of solar cell modules. Moreover, there has been concern that acetic acid gas and other unwanted gas generated by decomposition of EVA may affect the performance of the photovoltaic devices.
One approach to overcome the foregoing problems is to employ an encapsulant material for solar cells which is composed, at least partially, of ethylene/α-olefin copolymer. Also proposed is an encapsulant material for solar cells which contains an ethylene/α-olefin copolymer as a main component (see, e.g., Patent Literature 2). However, Patent Literature 2 fails to disclose any specific guide as to physical properties of ethylene/α-olefin copolymer for achieving preferable encapsulant material properties (e.g., heat resistance, transparency, flexibility, and process stability). A possible reason for this is that the technology disclosed therein presupposes conducting crosslinking treatment and therefore aims to achieve desired physical properties on the condition that the material undergoes crosslinking treatment.
Another disclosed encapsulant material for solar cells contains metallocene linear low-density polyethylene with a specific density range (see Patent Literature 3). Similarly, Patent Literature 3 fails to disclose, except for density, any specific guide as to physical properties for achieving preferable encapsulant material properties. A possible reason for this is that the technology disclosed therein presupposes conducting crosslinking treatment and therefore aims to achieve desired physical properties on the condition that crosslinking is conducted. Moreover, the encapsulant material disclosed by Patent Literature 3 has the drawback of possible reduction in heat resistance and humidity resistance.